This invention relates to an improved pressure saturator system which removes gases and excess saturant from a substrate or web during the pressure saturation process.
Pressure saturators are devices which impregnate a porous substrate such as paper with a liquid saturant. A wide variety of saturants are used, depending upon the physical or chemical properties desired for the impregnated substrate.
One common problem encountered in impregnating a porous substrate such as paper relates to the removal of air within the substrate. Such air tends to block the saturant from entering the substrate. If air is not allowed to leave the substrate properly, high pressure systems such as pressure rollers can trap the air within the substrate, and compress it. When pressure is released, any compressed air will re-expand and may tend to push the saturant back to the surface of the substrate. Certain dip tank saturators allow the air to dissolve into the saturant liquid. This approach can take a relatively long time to accomplish, which slows the saturating system.
Vacuum saturators have been proposed in which gases are first removed from the substrate before the substrate is immersed in the saturant. This approach has been used successfully; however suitable vacuum systems are relatively expensive to construct and operate. Furthermore, in practice a complete vacuum is typically unattainable, and some air remains within the substrate.
Menser U.S. Pat. No. 4,588,616 discloses an improved saturator system which employs a converging chamber defined by a mandrel and an opposed chamber defining element. The substrate to be saturated enters the chamber tightly wrapped around the mandrel. The saturant, located within the chamber, is impregnated into one surface of the substrate by hydrodynamic pressures created within the saturant by movement of the saturant and the substrate through the converging chamber. In the Menser saturator the mandrel is grooved to provide an escape path for gases from the substrate. As the saturant enters the substrate on one side, gases such as air exhaust out of the substrate through the opposite surface of the substrate. The grooves machined into the mandrel receive the air that leaves the substrate during saturation, and conduct this air circumferentially around the mandrel out of the chamber.
While the grooved mandrel of the Menser saturator has been shown to be very effective in use, limitations have been noted as well. The depth of the grooves in the mandrel influences the freedom with which gas is exhausted from the substrate, and therefore the volume of gases exhausted during the saturation process. Larger grooves allow a greater volume of gases to exhaust during saturation than do smaller grooves. Larger grooves however can cause distortion of the substrate, particularly when light weight substrates are processed. Such distortion can stretch and physically damage the substrate. For this reason, it may be appropriate in the Menser saturator to vary the width and depth of the grooves according to the weight of the substrate being processed and the depth of saturant penetration desired. However, since the grooves are machined into the mandrel surface, changing the depth of the grooves may necessitate changing mandrels, which is time consuming and expensive.
Another limitation of the Menser saturator is observed when operating parameters are chosen to impregnate the saturant completely through the substrate. Under these circumstances saturant emerges on the side of the substrate adjacent the mandrel, and this excess saturant can accumulate in the grooves of the mandrel. Such an accumulation of saturant reduces the depth of the grooves and therefore the flow of gases out of the substrate. Furthermore, such excess saturant can present a significant cleaning problem. With some saturants such as sodium silicate, any saturant that is allowed to dry on the mandrel is difficult to remove from the grooves.
It is an object of this invention to provide an improved saturator and saturating method which allow gas such as air in the substrate to be replaced with saturant right down to the fiber surfaces of the substrate. This is believed to provide optimal saturation, for the saturant is brought into intimate contact with the fibers of the substrate. For example, when the purpose of saturation is to create additional strength, it is desired that the saturant be impregnated into the substrate to reinforce the fiber junctions and the fiber walls.
It is a further object of this invention to provide a system that removes gases and any excess saturant from the side of the substrate adjacent the mandrel while minimizing or eliminating distortion of the substrate, and without requiring different mandrels for different weight substrates.
It is yet another object of this invention to provide a system which protects the mandrel from contact with excess saturant, thereby reducing mandrel cleaning requirements and allowing continuous, uninterrupted operation without accumulation of excess saturant on the mandrel.